Flow completion system

This application is a continuation of U.S. patent application Ser. No. 10/213,749 filed on Aug. 5, 2002, which is a continuation of U.S. patent application Ser. No. 09/815,437 filed on Mar. 22, 2001, now U.S. Pat. No. 6,494,257, which claims priority from U.S. Provisional Patent Application No. 60/192,124 filed on Mar. 24, 2000 and U.S. Provisional Patent Application No. 60/268,329 filed on Feb. 12, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a flow completion system for producing oil or gas from a subsea well. More particularly, the invention relates to such a system which comprises a tubing hanger which is supported in a tubing spool that is positioned over a well bore and two distinct pressure-containing barriers which are associated with the tubing hanger and which isolate the well bore from the external environment.

A typical horizontal-type flow completion system, such as that disclosed in U.S. Pat. No. 6,039,119, comprises a wellhead housing which is installed at the upper end of a well bore, a tubing spool which is connected to the top of the wellhead housing and which includes a central bore that extends axially therethrough, an annular tubing hanger which is suspended in the central bore, and a tree cap which is installed in the central bore above the tubing hanger. The tubing hanger supports at least one tubing string that extends into the well bore and defines a tubing annulus surrounding the tubing string. In addition, the tubing hanger comprises a concentric production bore which communicates with the tubing string and a lateral production passageway that extends between the production bore and a production outlet in the tubing spool. The tubing spool also includes an annulus passageway which extends from the tubing annulus to an annulus outlet, and a workover passageway which extends from the annulus passageway to a portion of the central bore that is located above the tubing hanger. The annulus and workover passageways provide for communication between the tubing annulus and the portion of the central bore located above the tubing hanger during installation and workover of the flow completion system.

The regulations of certain countries pertaining to the subsea production of oil and gas require that the flow completion system provide at least two pressure-containing barriers between the well bore and the environment at all times. In the production mode of operation of the typical horizontal flow completion system, the first barrier is provided by a wireline plug that is installed in the production bore above the production passageway, in conjunction with an annular, usually metal seal which is positioned between the tubing hanger and the tubing spool above the production outlet. The second barrier is provided by the tree cap, which is sealed to the tubing spool by an annular, typically metal seal and which often includes an axial through bore which in turn is sealed by a wireline plug or other suitable closure member.

During installation of the flow completion system, the tubing spool is landed onto the wellhead housing, after which a blowout preventer (“BOP”) is installed onto the tubing spool by means of a riser deployed from a surface vessel. The tubing hanger is then lowered on a tubing hanger running tool (“THRT”) through the riser and the BOP and landed in the central bore of the tubing spool. After the THRT is retrieved, the tree cap is lowered on a dedicated tool through the riser and the BOP and landed in the central bore directly above the tubing hanger. After the tree cap is installed, the THRT is retrieved, the BOP is retrieved, and the flow completion system is ready to be put into production. During a typical workover of the flow completion system, the BOP and the riser are once again connected to the tubing spool, the tree cap is usually removed from the tubing spool, and the THRT is connected to the tubing hanger. Once the workover operations are completed, the THRT is retrieved and the tree cap is re-installed through the riser and the BOP. Then the THRT is retrieved, the BOP is retrieved, and the flow completion system is ready to be put back into production.

Since the tree cap is required to maintain well pressure in the event of a failure of the first barrier, the tree cap typically comprises a rigid metal body and a robust metal lockdown mechanism to firmly lock the body to the tubing spool. Consequently, the tree cap is usually too heavy to be installed by a remotely operated vehicle (“ROV”) and must instead be lowered from the surface vessel on a specially designed tree cap running tool. Thus, installation of the tree cap requires a special running trip, both during installation of the flow completion system and after a workover operation. Each such trip typically requires a significant amount of valuable rig time to complete, which necessarily increases the cost of completing and maintaining the well.

In addition, during retrieval of the THRT prior to installing the tree cap, debris within the riser often falls into the central bore of the tubing spool above the tubing hanger. Left unattended, this debris could foul the sealing surfaces of the central bore and thereby prevent the tree cap from forming an effective seal with the tubing spool. Therefore, before the tree cap is installed the central bore must be thoroughly cleaned, a process that consumes additional valuable rig time and increases the cost of completing and maintaining the well.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a flow completion system for controlling the flow of fluid from a subsea well bore, the flow completion system comprising a tubing spool which includes a central bore that extends axially therethrough and a production outlet which communicates with the central bore, a tubing hanger which is supported in the central bore and which includes a production bore that extends axially therethrough and a production passageway that communicates between the production bore and the production outlet, the tubing hanger supporting a tubing string which extends into the well bore and defines a tubing annulus surrounding the tubing string, a first closure member which is positioned in the production bore above the production passageway, and a first annular seal which is positioned between the tubing hanger and the central bore above the production passageway. The first closure member and the first seal comprise a first pressure-containing barrier between the well bore and a surrounding environment. In addition, the flow completion system includes a second closure member which is positioned in the production bore above the first closure member, and a second annular seal which is positioned between the tubing hanger and the central bore above the first seal. The second closure member and the second seal comprise a second pressure-containing barrier between the well bore and the environment. Thus, both the first and the second barriers between the well bore and the environment are supported on the tubing hanger. Consequently, no need exists to provide a separate pressure-containing tree cap to seal the well bore from the environment.

These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of an embodiment of the flow completion system of the present invention;

FIG. 2 is an enlarged view of the top portion of the flow completion system shown in FIG. 1, but with the controls bridge component removed and with a running tool connected to the top of the tubing hanger component of the invention;

FIG. 3 is an enlarged sectional view of a tubing hanger anti-backoff mechanism which may be used in the present invention.

FIG. 4 is an enlarged sectional view of the production seal assembly component of the flow completion system shown in FIG. 1;

FIG. 5 is an enlarged sectional view of the secondary annular seal component of the flow completion system shown in FIG. 1;

FIG. 6 is a radial cross-sectional view of the tubing hanger component of the flow completion system shown in FIG. 1;

FIG. 7 is a longitudinal cross-sectional view of the tubing hanger component of FIG. 1 taken through the tubing hanger annulus bore and the annulus gate valve components of the invention;

FIG. 8 is an enlarged longitudinal cross-sectional view of the annulus gate valve component shown in FIG. 7;

FIG. 9 is a partial longitudinal cross-sectional view of another embodiment of a tubing hanger component of the present invention comprising an alternative annulus gate valve;

FIG. 10 is an enlarged sectional view of an alternative annulus closure member which is suitable for use with the present invention;

FIG. 11 is an enlarged sectional view of one embodiment of a service and controls conduit plug valve which is suitable for use with the present invention;

FIG. 12 is an enlarged sectional view of another embodiment of a service and controls conduit plug valve which is suitable for use with the present invention;

FIG. 13 is a longitudinal cross-sectional view of another embodiment of the flow completion system of the present invention;

FIG. 14 is a schematic representation of the flow completion system shown in FIG. 13;

FIG. 15 is an enlarged view of the top portion of the flow completion system shown in FIG. 14, but with the tree cap component removed and with a running tool connected to the top of the tubing hanger component of the invention;

FIG. 16 is a top view of the tree cap component of the flow completion apparatus shown in FIG. 13;

FIG. 16A is a cross-sectional view of the tree cap taken along line A—A of FIG. 16;

FIG. 16B is a cross-sectional view of the tree cap taken along line B—B of FIG. 16;

FIG. 16C is a cross-sectional view of the tree cap taken along line C—C of FIG. 16;

FIG. 17 is a top view of the locking cap component of the flow completion apparatus shown in FIG. 13; and

FIG. 18 is a longitudinal cross-sectional view of a further embodiment of the flow completion system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the flow completion assembly of the present invention, which is indicated generally by reference number <b>10</b>, is shown to comprise a tubing spool <b>12</b> which is connected to the top of a wellhead housing <b>14</b> that is installed at the upper end of a well bore (not shown), a tubing hanger <b>16</b> which is supported in the tubing spool <b>12</b> and from which is suspended at least one tubing string <b>18</b> that extends into the well bore, and an optional controls bridge <b>20</b> that is mounted to the top of the tubing spool <b>12</b>.

The tubing spool <b>12</b> is secured to the wellhead housing <b>14</b> by a suitable connector assembly <b>22</b> and is sealed to the top of the wellhead housing <b>14</b> with an appropriate annular seal <b>24</b>. The tubing spool comprises a generally cylindrical central bore <b>26</b> which extends axially therethrough, a lateral production outlet <b>28</b> which communicates with the central bore, and an annular support shoulder <b>30</b> which is located in the central bore. The support shoulder <b>30</b> may be either a separate support ring which is remotely installable in a corresponding receptacle in the central bore <b>26</b>, or an integral part of the tubing spool <b>12</b> which is formed by a reduced diameter portion of the central bore, as shown in FIG. <b>1</b>. As used herein, the term “tubing spool” should be understood to include a spool tree, a Christmas tree or any other spool member in which the tubing hanger may be supported.

The tubing hanger <b>16</b> is supported on the annular shoulder <b>30</b> and is attached to the upper end of the tubing string <b>18</b> by conventional means, such as threads. The tubing string <b>18</b> extends into the well bore and defines a production bore within the tubing string and a tubing annulus <b>32</b> surrounding the tubing string. While the invention is described herein with reference to a single tubing string <b>18</b>, it should be understood that tubing hanger <b>16</b> could be adapted to support two or more tubing strings, which may be either production or injection tubing strings, in a manner well known in the art.

The tubing hanger <b>16</b> is shown to include an annular main body <b>34</b> which comprises a generally stepped cylindrical outer wall <b>36</b>, a production bore <b>38</b> which extends axially and concentrically through the body <b>34</b> and communicates with the production bore of the tubing string <b>18</b>, a lateral production passageway <b>40</b> which extends between the production bore <b>38</b> and the outer wall <b>36</b>, and an annulus bore <b>42</b> which extends generally axially from the tubing annulus <b>32</b> to a portion of the central bore <b>26</b> that is located above the tubing hanger <b>16</b>. The tubing hanger <b>16</b> also includes a first closure member <b>44</b> and, in the embodiment of the invention depicted in FIG. 1, preferably also a second closure member <b>46</b> for controlling flow through the annulus bore <b>42</b>. The preferred embodiments of these closure members will be described in more detail below.

The tubing hanger <b>16</b> preferably also comprises a lower extension member or alignment sub <b>48</b> which includes an axial bore that is coaxial with the production bore <b>38</b> and is sufficiently large to allow the tubing string <b>18</b> to pass therethrough and connect directly to the body <b>34</b> of the tubing hanger. The extension member <b>48</b> is precisely oriented with respect to the tubing hanger via one or more alignment pins (not shown), and is secured thereto by any suitable means, such as a plurality of bolts (not shown). In addition, the extension member <b>48</b> preferably includes a radially extending lug or key <b>50</b> which, as the tubing hanger <b>16</b> is landed in the tubing spool <b>12</b>, engages a helical upper surface <b>52</b> of an orienting sleeve <b>54</b> to cause the tubing hanger <b>16</b> to rotate into a desired orientation. The orienting sleeve <b>54</b> is supported in an adapter bushing <b>56</b> which is connected to the lower portion of the tubing spool <b>12</b> by threads or other conventional means. In addition, suitable seals may be positioned around the adapter bushing <b>56</b> and between both the tubing spool <b>12</b> and the wellhead housing <b>14</b> to provide a backup to the seal <b>24</b>. Once the tubing hanger <b>16</b> reaches the proper orientation with respect to the tubing spool <b>12</b>, the key <b>50</b> will drop into a slot <b>58</b> which is formed in the orienting sleeve <b>54</b> at the bottom of the helical surface <b>52</b> to trap the tubing hanger into its proper orientation.

The flow completion assembly <b>10</b> may also include a lockdown sleeve <b>60</b> which is threaded onto the bottom of the adapter bushing <b>56</b>. After the tubing spool <b>12</b> is landed and locked on the wellhead housing <b>14</b>, the lockdown sleeve <b>60</b> may be unscrewed using an appropriate tool until a bottom surface of the lockdown sleeve engages the top of a production casing hanger (not shown) which is suspended in the wellhead housing <b>14</b>. The lockdown sleeve <b>60</b> will thus prevent the casing hanger from moving due to, for example, thermal expansion or contraction of the production casing. Suitable seals may also be provided between the lockdown sleeve <b>60</b> and both the adapter bushing <b>56</b> and the casing hanger so that the tubing spool <b>12</b> will, in effect, seal to the casing hanger in addition to or instead of the wellhead housing <b>14</b>.

Referring still to FIG. 1, the flow completion assembly <b>10</b> also includes at least one closure member for controlling flow through the production outlet <b>28</b>. In the embodiment of the invention depicted in FIG. 1, the production outlet closure member is housed in a valve block <b>62</b> which is bolted to the tubing spool <b>12</b>. However, as is well understood in the art, the closure member may be incorporated within the body <b>34</b> of the tubing spool <b>12</b> or comprise a separate, distinct component which is bolted or otherwise secured to the tubing spool <b>12</b>. The valve block <b>62</b> is shown to comprise an internal flow passageway <b>64</b> which communicates with the production outlet <b>28</b>, a production master valve <b>66</b>, a production wing valve <b>68</b>, and a flow loop <b>70</b> for connecting the flow passageway <b>64</b> to an external flowline (not shown). The flow completion assembly <b>10</b> may also comprise a number of service valve blocks <b>72</b>, each of which typically includes a remotely actuated valve <b>74</b> for selectively connecting an external service and control line (not shown) to the flow passageway <b>62</b>.

As shown in FIG. 2, the tubing hanger <b>16</b> is run into the tubing spool <b>12</b> on a running tool <b>76</b>, only a portion of which is shown. The running tool <b>76</b> is a relatively standard component and forms no part of the present invention; however, a description thereof is included herein to facilitate an understanding of the present invention. To simplify connecting the tubing hanger <b>16</b> to the running tool <b>76</b> and locking the tubing hanger to the tubing spool <b>12</b>, the tubing hanger is ideally provided with a cylindrical upper extension <b>78</b> which is threaded or secured by other suitable means to the top of body <b>34</b>, a locking mandrel <b>80</b> which is slidably received over the upper extension <b>78</b>, and an expandable locking ring <b>82</b> which is supported on a shoulder <b>84</b> that is formed by a reduction in the diameter of the outer wall <b>36</b>. The running tool <b>76</b> includes a tubular body <b>86</b> which is connected to a running string (not shown), an annular retention sleeve <b>88</b> which is secured to the upper end of the body <b>86</b>, a cylindrical first locking piston <b>90</b> which includes a first sleeve portion <b>92</b> that is slidably received over the body <b>86</b> below the retention sleeve <b>88</b>, and a cylindrical second locking piston <b>94</b> which includes a second sleeve portion <b>96</b> that is slidably received over the body <b>86</b> below the first sleeve portion <b>92</b>. A collet finger ring having a plurality of depending collet fingers <b>98</b> is attached to the lower end of the first locking piston <b>90</b>, and a wedge ring <b>100</b> is attached to the lower end of the second locking piston <b>94</b>.

In operation, the running tool <b>76</b> is lowered onto the tubing hanger <b>16</b> until the bottom of the first locking piston <b>90</b> engages the top of the locking mandrel <b>80</b> and the collet fingers <b>98</b> enter into a corresponding groove <b>102</b> that is formed on the inner diameter of the locking mandrel. The second locking piston <b>94</b> is then stroked downward to trap the collet fingers <b>98</b> in the groove <b>102</b> and thereby lock the first locking piston <b>90</b> to the locking mandrel <b>80</b>. Simultaneously, the wedge ring <b>100</b> will force an expandable locking ring <b>104</b> into a corresponding groove <b>106</b> that is formed on the inner diameter of the upper extension <b>78</b> to thereby lock the running tool <b>76</b> to the tubing hanger <b>16</b>. Once the tubing hanger is landed in the tubing spool <b>12</b>, the first locking piston <b>90</b> is stroked downward to push the locking mandrel <b>80</b> downward and force a lower nose ring <b>108</b> on the locking mandrel behind the lock ring <b>82</b>. This will push the lock ring <b>82</b> radially outwardly into a locking profile <b>110</b> which is formed in the central bore <b>26</b> to thereby lock the tubing hanger to the tubing spool <b>12</b>. The inner diameter surface of the nose ring <b>108</b> preferably comprises a plurality of serrations which will frictionally engage the outer wall <b>36</b> of the tubing hanger to aid in maintaining the locking mandrel <b>80</b> in the downward or locked position.

After the tubing hanger <b>16</b> is locked in the tubing spool, the second locking piston <b>94</b> is stroked upward to unlock the running tool <b>76</b> from the tubing hanger <b>16</b>. This will also unlock the collet fingers <b>98</b> from the locking mandrel <b>80</b> and therefore allow the running tool to be retrieved while the locking mandrel remains in the locked position. The tubing hanger <b>16</b> can also be retrieved using the running tool <b>76</b> by connecting the running tool to the tubing hanger as described above and then stroking the first locking piston <b>90</b> upward to pull the locking mandrel <b>80</b> upward and thereby allow the lock ring <b>82</b> to retract out of the locking profile <b>110</b>.

The tubing hanger <b>16</b> preferably includes an anti-backoff mechanism to maintain the locking mandrel <b>80</b> in the locked position and thereby ensure that the tubing hanger remains securely locked to the tubing spool <b>12</b> during operation of the flow completion assembly <b>10</b>. In the embodiment of the invention shown in FIG. 2, the anti-backoff mechanism comprises an anti-backoff ring <b>112</b> which is secured such as by threads to the upper extension <b>78</b> and includes a serrated outer surface that engages a corresponding grooved surface which is formed on the inner diameter of the locking mandrel <b>80</b>. This interface between the anti-backoff ring and the locking mandrel will thus maintain the locking mandrel in the locked position relative to the upper extension <b>78</b>.

An alternative anti-backoff mechanism is illustrated in FIG. <b>3</b>. In this embodiment, the anti-backoff mechanism, which is indicated generally by reference number <b>114</b>, is shown to comprise a split ring <b>116</b> which is supported in an annular recess <b>118</b> that is formed on the inner diameter of the locking mandrel <b>80</b>. The split ring <b>116</b> includes an integral upwardly directed tooth ring <b>120</b> which in the locking position of the anti-backoff mechanism <b>114</b> resides in one of a plurality of downwardly directed annular grooves <b>122</b> that are formed in the outer diameter of the upper extension <b>78</b> of the tubing hanger <b>16</b>. In this position, upward forces on the locking mandrel <b>80</b> are transmitted via the recess <b>118</b> through the split ring <b>116</b> and the tooth ring <b>120</b> to the upper extension <b>78</b>. Consequently, the locking mandrel will be prevented from moving upward and out of its locking position with respect to the tubing hanger <b>16</b>. However downward forces on the locking mandrel <b>80</b> will cause the tooth ring <b>120</b> to cam out of its groove <b>122</b> and engage in a lower groove <b>122</b>. The anti-backoff mechanism <b>114</b> may also comprise a plurality of guide pins <b>124</b> which extend laterally into corresponding holes <b>126</b> that are formed in the split ring <b>116</b> to help maintain the split ring properly oriented within the recess <b>118</b>.

Furthermore, although the split ring <b>116</b> is normally biased against the upper extension <b>78</b>, the anti-backoff mechanism <b>114</b> preferably includes a plurality of release pins <b>128</b> to urge the tooth ring <b>120</b> away from the grooves <b>122</b> so that the locking mandrel <b>80</b> can be retracted during retrieval of the tubing hanger <b>16</b>. Each release pin <b>128</b> comprises cylindrical body portion having an enlarged diameter head <b>130</b> which is trapped in a corresponding stepped-diameter hole <b>132</b> that is formed in the upper extension <b>78</b>. Prior to retrieval of the tubing hanger <b>16</b>, the running tool <b>76</b> is landed on the tubing hanger and the second locking piston <b>94</b> is stroked downward to bring an outer diameter portion of the second locking piston against a rear camming face <b>134</b> of each release pin <b>128</b>. This forces the release pins <b>128</b> radially outwardly into split ring <b>116</b> and pushes split ring away from the upper extension <b>78</b> to thereby move the tooth ring <b>120</b> out of engagement with the grooves <b>122</b>.

Referring again to FIG. 1, in the production mode of operation of the flow completion system <b>10</b>, the production bore <b>38</b> is sealed above the production passageway <b>40</b> by a first closure member <b>136</b> and, in accordance with the present invention, preferably also a second closure member <b>138</b>. The closure members <b>136</b>, <b>138</b> are preferably wireline crown plugs which are received in corresponding profiles that are formed in the production bore <b>38</b> or in a bore insert (not shown) that is secured and sealed within production bore. In addition, a test port (not shown) is ideally routed between the plugs in a conventional fashion to prevent the creation of a hydraulic lock during installation of the second plug <b>138</b> and to facilitate the testing of both plugs after they are installed. Furthermore, the tubing hanger <b>16</b> is sealed to the central bore <b>26</b> of the tubing spool <b>12</b> by a first annular seal <b>140</b> which is positioned between the tubing hanger and the tubing spool above the production passageway <b>40</b> and, in accordance with the present invention, preferably also a second annular seal <b>142</b> which is positioned between the tubing hanger and the tubing spool above the first seal <b>140</b>.

Referring to FIG. 4, the first seal <b>140</b> ideally forms part of a production seal assembly <b>144</b> that also includes an annular lower seal <b>146</b> which is positioned between the tubing hanger <b>16</b> and the tubing spool <b>12</b> below the production passageway <b>40</b>. Each seal <b>140</b>, <b>146</b> is preferably a straight bore-type metal seal which comprises an inner radial sealing lip <b>148</b> that engages a corresponding annular sealing surface <b>150</b> which is formed on the outer wall <b>36</b> of the tubing hanger and an outer radial sealing lip <b>152</b> that engages a corresponding annular sealing surface <b>154</b> which is formed on the central bore <b>26</b>. In addition, the seals <b>140</b>, <b>146</b> are optimally oriented so that the sealing lips will be energized into engagement with their corresponding sealing surfaces by the pressure within the production passageway <b>40</b>. The seals <b>140</b>, <b>146</b> are spaced apart on the tubing hanger <b>16</b> by a spacer ring <b>156</b> which includes a lateral hole <b>158</b> that aligns with the production passageway <b>40</b>, and the entire seal assembly <b>144</b> is secured to the tubing hanger by a retainer ring <b>160</b>. Furthermore, in the embodiment of the invention shown in FIG. 4, each seal <b>140</b>, <b>146</b> includes a pair of backup seal rings <b>162</b>, <b>164</b> which are mounted in corresponding grooves that are formed on the seals <b>140</b>, <b>146</b>. While, the backup seal rings <b>162</b>, <b>164</b> may be any suitable seals, they are preferably non-metallic face-type seals.

The second seal <b>142</b> is preferably similar in design an operation to the first seal <b>140</b> and is connected to the outer wall <b>36</b> using a suitable T-support ring. Alternatively, as shown in FIG. 5 the second seal <b>142</b> may comprise a depending radial leg <b>166</b> on which is formed an annular sealing lip <b>168</b> that engages a corresponding sealing surface <b>170</b> which is formed in the central bore <b>26</b>. The seal <b>142</b> is oriented such that pressure in the central bore <b>26</b> below the seal will tend to force the leg <b>166</b> radially outward and thus the sealing lip <b>168</b> into tighter sealing engagement with the sealing surface <b>170</b>. The seal <b>142</b> is preferably secured to the outer wall <b>36</b> of the tubing hanger <b>16</b> by threads <b>172</b>. When the seal <b>142</b> is tightened against the tubing hanger <b>16</b>, a top surface <b>174</b> of the seal will sealingly engage a step <b>176</b> that is formed in the outer wall <b>36</b>. In addition, a preferably a C-shaped seal ring <b>178</b> may be positioned in a corresponding groove that is formed in the top surface <b>174</b> to provide an additional pressure barrier between the seal <b>142</b> and the tubing hanger <b>16</b>. Furthermore, the flow completion system <b>10</b> may include at least one backup seal ring <b>180</b> for sealing between the tubing hanger <b>16</b> and the tubing spool <b>12</b>. The seal ring <b>180</b>, which is preferably a non-metallic S-type radial interference seal, is positioned in a corresponding annular recess that is formed in the outer wall <b>36</b> above the seal <b>142</b>.

Thus, in the production mode of operation of the flow completion system <b>10</b>, the tubing hanger supports both of the industry required first and second pressure containing barriers between the production bore and the environment. The first barrier is provided by the first plug <b>136</b> and the first seal <b>140</b>, and the second barrier is provided by the second plug <b>138</b> and the second seal <b>142</b>. It should be understood that the function of the first and second seals is to isolate the production bore from the environment. Therefore, either the first seal <b>140</b> or the second seal <b>142</b>, or both, may comprise an annular ring seal which is positioned between the tubing hanger and the central bore concentric with the production passageway and the production outlet. An exemplary such seal is disclosed in U.S. Pat. No. 5,868,204, which is hereby incorporated herein by reference.

Therefore, a separate pressure-containing tree cap is not required to provide the second barrier. Thus, to the extent required by a specific application, a lightweight, ROV deployable tree cap may be used with the flow completion system <b>10</b>. Moreover, once the tubing hanger <b>16</b> is landed in the tubing spool <b>12</b> and the plugs <b>136</b>, <b>138</b> are installed, the BOP and riser which are used during installation of the tubing hanger may be removed.

In the embodiment of the flow completion system <b>10</b> depicted in FIG. 1, the tubing hanger <b>16</b> also comprises a number of service and control conduits <b>182</b>. As shown in FIG. 6, the service and control conduits <b>182</b> are arranged radially around the central axis of the tubing hanger <b>16</b> and extend from the top of the tubing hanger generally vertically into or completely through the tubing hanger. The service and control conduits <b>182</b> provide for the communication of cables or fluids through the tubing hanger between corresponding external service and control lines (not shown) and devices or positions located in or below the tubing hanger. For example, one or more service and control conduits <b>182</b> may communicate hydraulic control fluid from a subsea control module to an operational device, such as a surface controlled subsea safety valve (not shown), which is located in the tubing string. Also, one or more service and control conduits <b>182</b> may communicate pressure from the tubing annulus <b>32</b> to an external service and control line which is connected to a pressure monitoring device located, for instance, on a surface vessel. In this context, the annulus bore <b>42</b> may be considered to be a service and control conduit.

The controls bridge <b>20</b> shown in FIG. 1 may be used in the flow completion system <b>10</b> to facilitate the connection of the service and control conduits <b>182</b> to their corresponding external service and control lines through the top of the tubing hanger <b>16</b>. To this end the controls bridge <b>20</b>, which is described more fully in applicants' co-pending U.S. patent application Ser. No. 09/815,431, which is hereby incorporated herein by reference, includes a number of internal bridge lines and an actuating mechanism for remotely connecting each bridge line to both a service and control conduit in the tubing hanger and a corresponding external service and control line. Consequently, the need to make these connections individually or radially through the tubing spool <b>12</b> is eliminated. In addition, the controls bridge may include one or more closure members for controlling flow through respective bridge lines, thereby eliminating the need to include these closure members on the tubing spool <b>12</b> or in the tubing hanger <b>16</b>. Additionally, the controls bridge <b>20</b> is preferably sufficiently lightweight to be installed and retrieved using an ROV.

As mentioned above, the tubing hanger <b>16</b> includes at least a first closure member <b>44</b> and preferably also a second closure member <b>46</b> for controlling flow through the annulus bore <b>42</b>. At least one of these closure members is preferably an active closure member, that is, one that is actuated in at least one direction by an external power source. In this respect, exemplary active closure members which may be suitable for use in the present invention include hydraulically, electrically or manually actuated ball valves, plug valves, sleeve valves, gate valves, butterfly valves or stinger valves. The other closure member may be either an active closure member or a passive closure member, the latter typically being defined as one that is actuated in one direction by the fluid contained by the closure member or by a component with which the closure member is assembled, and in the opposite direction by a return biasing means, such as a spring. Any of a variety of passive closure members may be appropriate for use with the present invention, including, but not limited to, check valves, poppet valves and flapper valves.

In accordance with a preferred embodiment of the invention, the first closure member <b>44</b> is preferably an internal gate valve which is similar to that disclosed in applicants' co-pending U.S. patent application Ser. No. 09/815,436, which is hereby incorporated herein by reference. Referring to FIGS. 6-8, the gate valve <b>44</b> is unique in that substantially all of its operational components are housed entirely within the body <b>34</b> of the tubing hanger <b>16</b>. In addition, the gate valve <b>44</b> is oriented generally axially so as to occupy a minimum of the radial cross sectional area of the tubing hanger <b>16</b>. In order to most readily accommodate this vertical orientation of the gate valve <b>44</b>, the annulus bore preferably includes a lateral branch which is connected to a longitudinal branch, and the gate valve is disposed across the lateral branch. For example, in FIGS. 7 and 8 the annulus bore <b>42</b> is shown to comprise an upper branch <b>184</b> which extends generally axially through the body <b>34</b> to the top of the tubing hanger <b>16</b>, a lower branch <b>186</b> which extends generally axially through the body to the bottom of the tubing hanger, and an intermediate branch <b>188</b> which extends generally laterally between the upper and lower branches. To facilitate the formation of the annulus bore <b>42</b>, the intermediate branch <b>188</b> is ideally machined into the outer wall <b>36</b> and then sealed by a plug member <b>190</b> or any other suitable means.

The gate valve <b>44</b> comprises a generally rectangular gate cavity <b>192</b> which extends generally laterally through the outer wall <b>36</b> and intersects both the intermediate branch <b>188</b> and a service and control conduit <b>182</b><i>a</i>. In addition, an annular seat pocket <b>194</b> extends transversely into the body <b>34</b> from each intersection of the gate cavity <b>192</b> with the intermediate branch <b>188</b>. The gate valve <b>44</b> also comprises two ring-shaped floating-type seats <b>196</b>, each of which is positioned in a seat pocket <b>194</b>, a gate <b>198</b> which is slidably disposed between the seats <b>196</b>, and an actuating mechanism <b>200</b> which is positioned substantially in the service and control conduit <b>182</b><i>a. </i>

The actuating mechanism <b>200</b> functions to move the gate <b>198</b> between a valve open position in which a lateral hole <b>202</b> in the gate is aligned with the intermediate branch <b>188</b>, and a valve closed position in which the hole <b>202</b> is offset from the intermediate branch <b>188</b>, as shown in FIG. <b>7</b>. In a preferred embodiment of the invention, the actuating mechanism <b>200</b> comprises an upper piston head <b>204</b> which supports a seal <b>206</b> that engages the service and control conduit <b>182</b><i>a</i>, an elongated piston rod <b>208</b> which is connected to the bottom of the piston head <b>204</b>, a valve stem <b>210</b> which is connected between the lower end of the piston rod <b>208</b> and the top of the gate <b>198</b>, for example via a T-slot connection <b>212</b>, and a return biasing mechanism, which in the embodiment of the invention shown in FIGS. 7 and 8 is a mechanical biasing means such as a stack of Belleville washers <b>214</b>. In addition, the valve stem <b>210</b> is preferably sealed against the service and control conduit <b>182</b><i>a </i>by a suitable packing <b>216</b>, which is held in place by a gland nut <b>218</b> that in turn is secured to the body <b>34</b> by suitable means, such as a retainer screw <b>220</b>.

The opening that the gate cavity <b>192</b> forms in the wall <b>36</b> is optimally closed by a simple cover plate <b>222</b> which is held in place by a cylindrical sleeve <b>224</b> that is telescopically received over the tubing hanger <b>16</b>. The pressure in the gate cavity <b>192</b> is therefore preferably contained within the first and second seals <b>140</b>, <b>142</b> which are used to seal the tubing hanger to the tubing spool, in addition to the packing <b>216</b>. Consequently, no need exists for separate sealing means between the body <b>34</b> and either the cover plate <b>222</b> or the sleeve <b>224</b> to contain the pressure within the gate cavity <b>192</b>. However, the present invention contemplates that one or more such seals could be provided between the body <b>34</b> and either the cover plate <b>22</b> or the sleeve <b>224</b>, to contain the pressure within the gate cavity <b>192</b>, especially when the opening that the gate cavity makes in the wall <b>36</b> is not located between the seals <b>140</b>, <b>142</b>. In addition, instead of the cover plate <b>222</b> being retained by the sleeve <b>224</b>, the sleeve could be dispensed with and the cover plate simply bolted onto the body, in which event seals are preferably provided between the cover plate and the body <b>34</b> to retain the pressure within the gate cavity <b>192</b>.

In operation, the gate valve <b>44</b> is normally in the closed position. When it is desired to open the annulus bore <b>42</b>, a pressure sufficient to overcome the combined force of the return biasing means <b>200</b> and the friction at the various interfaces of the gate valve is introduced into the service and control conduit <b>182</b><i>a </i>above the piston head <b>204</b>. As shown in FIG. 7, a male coupling <b>226</b> may be installed in the top of the service and control conduit <b>182</b><i>a </i>to facilitate connecting a pressure source, such as a high pressure hydraulic fluid source, to the service and control conduit. The male coupling <b>226</b> is engaged by a corresponding female coupling (not shown) which is mounted in either the running tool <b>76</b> or the controls bridge <b>20</b> and which in turn is connected to the pressure source by a corresponding external service and control line in a conventional fashion. Alternatively, the top of the service and control conduit <b>182</b><i>a </i>may merely include a seal profile for a stab which is carried on the running tool or the controls bridge and is connected to the pressure source by conventional means. The pressure in the service and control conduit <b>182</b><i>a </i>will force the piston head <b>204</b> downward and thus move the gate <b>198</b> into the open position. In this position, fluid in the tubing annulus <b>42</b> is allowed to flow from the lower branch <b>186</b>, through the intermediate branch <b>188</b> and into the upper branch <b>184</b>, where it will encounter the closure member <b>46</b> if present. As shown in FIG. 7, the tubing hanger <b>16</b> preferably includes a compensation port <b>228</b> which extends between the top of the tubing hanger and a portion of the service and control conduit <b>182</b><i>a </i>that is located below the piston head <b>204</b>. While not shown in the drawings, a male coupling or stab profile may be provided at the top of the compensation port <b>228</b> to facilitate the connection of this port through the running tool or the controls bridge with a corresponding external service and control line.

When it is desired to close the gate valve <b>44</b>, the pressure is removed from the service and control conduit <b>182</b><i>a</i>, whereupon the force from the return biasing mechanism <b>214</b> combined with the pressure in the annulus bore <b>42</b> acting on the stem <b>210</b> will push the piston head <b>204</b> upward and move the gate <b>198</b> into the closed position. If the means supplying the pressure to the service and control conduit <b>182</b><i>a </i>should fail for any reason, the return biasing mechanism <b>214</b> will either retain the gate <b>198</b> in the closed position or move the gate from the open position to the closed position. Thus, in the preferred embodiment of the invention the gate valve <b>44</b> is a “fail closed” device.

In an alternative embodiment of the gate valve <b>44</b> which is not specifically illustrated in the drawings, the actuating mechanism <b>200</b> is a pressure balanced operating mechanism. Consequently, the return biasing mechanism <b>214</b> would not be required. Instead, the compensation port <b>228</b> is connected to a pressure source in the same manner that the service and control conduit <b>182</b><i>a </i>is connected to a pressure source. In order to return the gate valve to the closed position, therefore, pressure is introduced into the compensation port <b>228</b> to force the piston head <b>204</b>, and thus the gate <b>198</b>, upward. In this embodiment, the gate valve <b>44</b> would be a “fail as is” device.

Referring now to FIG. 9, an alternative gate valve <b>44</b>′ which is suitable for use with the present invention is shown installed in a tubing hanger <b>16</b>′. In this embodiment the tubing hanger <b>16</b>′ is shown to comprise an annulus bore <b>42</b> having a first branch <b>230</b> which extends generally laterally through the tubing hanger from the tubing annulus <b>32</b>, and a second branch <b>232</b> which extends from the first branch to the top of the tubing hanger. In addition, the gate valve <b>44</b>′ comprises a gate cavity <b>192</b> that extends laterally through the wall <b>36</b> of the tubing hanger generally coaxially with the first branch <b>230</b>. The gate cavity <b>192</b> forms an opening <b>234</b> in the wall <b>36</b> which is preferably closed by a cover <b>236</b> that is ideally removably attached to the tubing hanger using any suitable means, such as bolts (not shown). In addition, the cover <b>236</b> is optimally sealed to the tubing hanger with at least one annular seal <b>238</b>.

In the embodiment of the invention depicted in FIG. 9, the gate valve <b>44</b>′ is shown to comprise a gate <b>198</b> which is slidably disposed across the first branch <b>230</b> between a pair of seats <b>240</b>, <b>242</b>. The first seat <b>240</b> is similar to the seats <b>196</b> discussed above. The second seat <b>242</b> can be identical to the first seat <b>240</b> or, as shown in FIG. 9, it can comprise an annular body which is attached to or formed integrally with the cover <b>236</b>. In either event, the cover <b>236</b> preferably includes a port <b>244</b> which aligns with the through bores in the seats <b>240</b>, <b>242</b> to define a flow passage <b>246</b> through the gate valve <b>198</b> which extends between the tubing annulus <b>32</b> and the first branch <b>232</b>.

The gate valve <b>44</b>′ further includes an actuating mechanism to move the gate <b>198</b> between a closed position, in which a lateral hole <b>202</b> in the gate is offset from the flow passage <b>246</b>, to an open position, in which the hole <b>202</b> is aligned with the flow passage, as shown in FIG. <b>9</b>. The actuating mechanism (not shown) is positioned in a service and control conduit <b>182</b><i>a </i>that is oriented generally vertically over the gate <b>198</b>. In addition, the actuating mechanism, which can be similar to any of the actuating mechanisms discussed above, is connected to the gate <b>198</b> via a valve stem <b>210</b>.

Although not illustrated in FIG. 9, it should be understood that other configurations of the gate valve are within the scope of the present invention. For example, the gate cavity could extend longitudinally into the tubing hanger from the bottom thereof. Thus, the gate cavity would intersect the first branch and provide a convenient means for installing the seats in the first branch. In this example, the cover which is used to close the opening that the gate cavity makes in the bottom of the tubing hanger would not include a port. Rather, the first branch would communicate directly with the tubing annulus through the wall of the tubing hanger.

Also, although not depicted in the Figures, other configurations of the annulus bore <b>42</b> are considered to be within the scope of the present invention. For example, the first branch of the annulus bore could extend generally laterally through the tubing hanger and communicate with the portion of the central bore of the tubing spool that is located above the seals which are employed to seal the tubing hanger to the tubing spool. In this example, the second branch would extend generally longitudinally from the first bore down through the tubing hanger to the tubing annulus. In addition, the gate valve would preferably be installed in or near the top of the tubing hanger.

Referring again to FIG. 7, the closure member <b>46</b> is preferably a sting-open check valve which is mounted in the upper branch <b>184</b> of the annulus bore <b>42</b>. The check valve <b>46</b> comprises an annular head <b>248</b> which is adapted to seal against a corresponding conical seat <b>250</b> that is formed in the upper branch <b>184</b>, a coil spring <b>252</b> which is preferably supported in a collet <b>254</b> that in turn is mounted in the upper branch, a stem <b>256</b> which is connected to the head and which extends through the upper branch toward the top of the tubing hanger <b>16</b>, and at least one radial stabilizer fin <b>258</b> which is secured to the stem above the head. The spring <b>252</b> urges the head <b>248</b> into sealing engagement with the seat <b>250</b> to thereby close the upper branch <b>184</b>. In order to open the upper branch, a stinger or similar member (not shown) is inserted into the top of the upper branch and against the stabilizer fin <b>258</b> to thereby push the head <b>248</b> off of the seat and into the open position. In order to facilitate the assembly of the check valve <b>46</b>, the outer wall <b>34</b> of the tubing hanger <b>16</b> may include a lateral opening which is sealed by a preferably removable plug <b>260</b>.

As an alternative to the closure member <b>46</b>, the flow completion system <b>10</b> may comprise a simple debris valve which is mounted in the top of the upper branch <b>184</b> to prevent debris from falling into the annulus bore when the tubing hanger running tool is removed from the tubing hanger but allow fluid to pass through the upper branch when the annulus bore is connected to an external conduit, such as an external service and control line. The construction and operation of the debris valve are explained more fully in applicants' co-pending U.S. patent application Ser. No. 09/815,436.

FIGS. 10-12 illustrate three alternative closure members which may be used to control flow through the annulus bore <b>46</b> or the service and control conduits <b>182</b>. The closure member <b>262</b> shown in FIG. 10 is an annulus plug valve which comprises a cylindrical sleeve <b>264</b> that is secured in the bottom of the annulus bore <b>42</b>, a head <b>266</b> which is slidably received in the bottom of the annulus bore above the sleeve, a stem <b>268</b> which extends through the sleeve between the head and a flange <b>270</b>, and a coil spring <b>272</b> which is operatively engaged between the bottom of the sleeve and the flange to urge the head against the top of the sleeve. The head <b>266</b> supports two pairs of axially spaced apart seals <b>274</b>, <b>276</b> that seal against the annulus bore <b>42</b>. In addition, a third pair of seals <b>278</b> is positioned between the sleeve <b>264</b> and the annulus bore and a fourth pair of seals <b>280</b> is positioned between the stem <b>268</b> and the sleeve <b>264</b>. In operation, hydraulic pressure is introduced through a port <b>282</b> and into a piston chamber which is formed between the seals <b>274</b>, <b>278</b> and <b>280</b> to urge the head <b>266</b> in to the closed position, in which the seals <b>274</b>, <b>276</b> will straddle an inlet <b>284</b> between the tubing annulus and the annulus bore <b>42</b> to thereby close the annulus bore <b>42</b>. In the absence of hydraulic pressure in the piston chamber, the spring will force the head <b>266</b> downward into the its open position. Thus, the plug valve <b>262</b> is preferably a “fail open” device.

The closure member <b>286</b> shown in FIG. 11 is, for example, a chemical injection valve that is used to control the flow of a chemical through a corresponding service and control conduit <b>182</b>. The valve <b>286</b> comprises a support sleeve <b>288</b> that is secured and sealed in the bottom of a service and control conduit <b>182</b>, a cylindrical seal body <b>290</b> which is slidably received within both the conduit <b>182</b> and the sleeve <b>288</b> and which includes a conical sealing head <b>292</b> that engages a corresponding seat <b>294</b> which is formed in the conduit <b>182</b>, a coil spring <b>296</b> which is operatively engaged between the sleeve <b>288</b> and the body <b>290</b> to urge the head into sealing engagement with the seat, and a suitable fitting <b>298</b> which is attached to the seat and through which a conduit that extends down hole may be connected to the valve <b>286</b>. In addition, the valve <b>286</b> includes at least one first seal <b>300</b> which is positioned between the body <b>290</b> and the conduit <b>182</b>, at least one second seal <b>302</b> which is positioned between the body and the sleeve, and at least one third seal <b>306</b> which is mounted between the sleeve and the body <b>34</b> of the tubing hanger <b>16</b>. In operation, hydraulic pressure is introduced through a port <b>306</b> and into a piston chamber that is formed between the seals <b>300</b>, <b>302</b> and <b>304</b> to force the body <b>290</b> downward, which will pull the head <b>292</b> off of the seat <b>294</b> and thereby open the valve <b>286</b>. In this position, fluid in the conduit <b>182</b> will flow past the head <b>292</b>, into an expansion <b>308</b> which is formed in the conduit <b>182</b>, through a number of inlets <b>310</b> that are formed in the body <b>290</b> below the head, through the body, through the sleeve <b>288</b>, and out through the fitting <b>298</b>. In the absence of hydraulic pressure in the piston chamber, the spring <b>296</b> will force the body <b>290</b> into the closed position. Thus, the valve <b>286</b> is a “fail closed” device. Moreover, in this condition the valve <b>286</b> may be opened by applying sufficient pressure in the conduit <b>182</b> above the valve to overcome the force of the spring <b>296</b>.

The closure member <b>312</b> depicted in FIG. 12 is shown to be similar in many respects to the valve <b>286</b> discussed above. However, in valve <b>312</b> a radial flange <b>314</b> is attached to the lower portion of the seal body <b>290</b>, and the spring <b>296</b> is operatively engaged between the sleeve <b>288</b> and the flange to urge the seal body downward. This will pull the sealing head <b>292</b> off of the seat <b>294</b> and into the open position shown in FIG. <b>12</b>. In addition, instead of the expansion <b>308</b> in the conduit <b>182</b>, the upper diameter of the body <b>290</b> is reduced as at <b>316</b> to allow fluid in the conduit <b>182</b> to flow past the head <b>292</b> and into the inlets <b>310</b>. Moreover, the valve <b>312</b> is preferably a “pilot to close” valve; therefore, if pressurized fluid exists within the conduit that is connected to the fitting <b>298</b>, the plug valve will remain in the closed position. Also, in the event of a failure of the means for supplying hydraulic pressure to the valve <b>312</b>, the spring <b>296</b> will maintain the body <b>290</b> in the open position. Thus, the valve <b>312</b> is a “fail open” device.

In order to provide an effective barrier between the well bore and the environment, the tubing hanger <b>16</b> preferably includes one of the aforementioned closure members to control the flow through each service and control conduit <b>182</b> that extends completely through the tubing hanger to other than a down hole valve. The closure member will therefore act as an initial barrier between the well bore and the environment through the service and control conduit. This barrier is in addition to the barrier which is provided by a conventional fluid coupling that is typically installed in the service and control conduit. As is known in the art, the conventional fluid coupling includes a poppet-type valve which will close when the coupling is disengaged from a corresponding coupling that is installed in a tubing hanger running tool, a controls bridge or a radial penetrator. As an alternative to employing an individual closure member in each service and control conduit <b>182</b>, the flow completion assembly <b>10</b> may comprise a multiport gate valve assembly to control the flow through a number of service and control conduits simultaneously. Such a gate valve assembly, which is shown as <b>318</b> in FIG. 1, is described in applicants' co-pending U.S. patent application Ser. No. 09/815,395, which is hereby incorporated herein by reference. With closure members such as the above in place in the service and control conduits <b>182</b>, the tubing spool <b>16</b> will contain both of the industry required first and second barriers between the well bore and the environment.

Referring now to FIG. 13, another embodiment of a flow completion system of the present invention is shown which is similar in many respects to the flow completion system <b>10</b> described above. However, the flow completion system of this embodiment, which is indicated generally by reference number <b>410</b>, also comprises an annulus passageway <b>412</b> which extends through the tubing spool <b>12</b> between the tubing annulus <b>32</b> and an annulus outlet <b>414</b>, an annulus master valve <b>416</b> for controlling flow through the annulus passageway, an annulus wing valve <b>418</b> for controlling flow through the annulus outlet, a crossover line <b>420</b> which extends from the annulus outlet to a portion of the flow passageway <b>64</b> that is located between the production valves <b>66</b> and <b>68</b>, and a crossover valve <b>422</b> for controlling flow through the crossover line. The valves <b>416</b>, <b>418</b> and <b>422</b> may be any of the closure members discussed above, but they are preferably remotely operable gate valves. In addition, the valves may be mounted in the tubing spool <b>12</b>, mounted in one or more valve blocks that are attached to the tubing spool, or mounted individually on the tubing spool. With the above-described arrangement of the flow completion system <b>410</b>, the pressure within the tubing annulus <b>32</b> can be monitored through the annulus passageway <b>412</b> during production, and fluids can be circulated between the production bore <b>38</b> and the tubing annulus through the annulus passageway and the crossover line <b>420</b> during installation and workover.

The flow completion system <b>410</b> may also include one or more male radial penetrator couplings <b>424</b>, each of which is connected to a corresponding service and control conduit <b>182</b> within the tubing hanger <b>16</b>. The male couplings <b>424</b> are mounted on the outer wall <b>36</b> of the tubing hanger in a conventional fashion, and each male coupling is releasably connectable with a corresponding female coupling (not shown) which is carried by a conventional radial penetrator assembly that is mounted on the tubing spool <b>12</b>. Thus, when the tubing hanger is landed on the tubing spool, the penetrator assembly may be actuated to bring the female couplings into engagement with their corresponding male couplings <b>424</b> to thereby establish communication between the service and control conduits <b>182</b> in the tubing hanger <b>16</b> and the external service and control lines to which the female couplings are connected.

In addition, as shown schematically in FIG. 14, one or more of the service and control conduits <b>182</b> which is connected to a radial penetrator coupling <b>424</b> may be routed within the body <b>34</b> of the tubing hanger <b>16</b> to a corresponding axial service and control conduit <b>182</b> that enters from the top of the tubing hanger. In this manner, a device with which a tubing hanger running tool communicates during installation of the tubing hanger <b>16</b>, for example a surface controlled subsea safety valve (“SCSSV”), can be monitored during installation of the tubing hanger and then connected to an external service and control line through the radial penetrator once the running tool is disconnected from the tubing hanger. A conventional poppet-type fluid coupling may be installed in each vertical service and control conduit <b>182</b> to seal the conduit once the running tool is disconnected. Alternatively, a fluid coupling <b>426</b> which comprising both a poppet valve and a gate valve may be employed in each such service and control conduit. Such a coupling, which is described more fully in applicants' co-pending U.S. patent application Ser. No. 09/844,579, which is hereby incorporated herein by reference, will provide two barriers between the well bore and the environment through the service and control conduit.

Referring to FIG. 15, the tubing hanger <b>16</b> of the flow completion system <b>410</b> is preferably installed using a tubing hanger running tool <b>428</b>. The running tool <b>428</b> is similar in most respects to the running tool <b>76</b> discussed above, including in the means and manner by which the running tool is connected to the tubing hanger <b>16</b>. However, the running tool <b>428</b> also includes a tubular member <b>430</b> which is secured within the body <b>86</b> such as by threads <b>432</b>. The tubular member <b>430</b> defines a production port <b>434</b> within the running tool <b>428</b> which communicates with a riser (not shown) in a manner well understood by those of skill in the art. In addition, the bottom of the tubular member <b>430</b> forms a production seal stab <b>436</b> which, when the running tool is connected to the tubing hanger <b>16</b>, is received in the top of the production bore <b>38</b> to connect the production port <b>434</b> with the production bore.

Furthermore, when the running tool <b>428</b> is connected to the tubing hanger <b>16</b>, the bottom of the body <b>86</b> is spaced apart from the top of the tubing hanger to thereby form an annular gallery <b>438</b> which is in communication with the tubing hanger annulus bore <b>42</b>. The gallery <b>438</b> is sealed from the environment by both a lower annular isolation seal <b>440</b>, which is engaged between the tubing hanger <b>16</b> and the upper extension <b>78</b>, and an upper annular isolation seal <b>442</b>, which is engaged between the upper extension and the outer diameter of the wedge ring <b>100</b>. The seals <b>440</b>, <b>442</b> may be any suitable seals, but are preferably non-metallic face seals. Also, the outer diameter of the tubular member <b>430</b> is designed to be smaller than the inner diameter of the body <b>86</b> in order to form an annular volume or “annulus port” <b>444</b> between these two members which extends between the gallery <b>438</b> and the top of the body. The spacing between the tubular member and the body is ideally maintained by a number of fluted centralizers <b>446</b> which may be attached to or formed integrally with either the tubular member or the body. Moreover, the threads <b>432</b> which secure tubular member <b>430</b> to the body <b>86</b> are fluted to allow for fluid to pass through this connection. In this manner, when a BOP (not shown) is connected to the tubing spool <b>12</b> and the BOP rams are closed around the tubular member <b>430</b> or the retention sleeve <b>88</b>, fluid communication between the BOP choke and kill line and the tubing hanger annulus bore <b>42</b> may be established through the annulus port <b>444</b> and the gallery <b>438</b>.

Referring again to FIG. 13, the flow completion system <b>410</b> may optionally comprise a single “dual-sealing” wireline plug <b>448</b> to seal the production bore <b>38</b> above the production passageway <b>40</b>. The dual-sealing plug <b>448</b> comprises a wireline deployable plug body <b>450</b> which is removably connectable within a bore insert <b>452</b> that is secured and sealed in the production bore <b>38</b>. In addition, the dual-sealing plug <b>448</b> includes a first annular sealing assembly <b>454</b> for sealing between the body <b>450</b> and the bore insert <b>452</b>, and a second annular sealing assembly <b>456</b> for sealing between the body and, preferably, the production bore <b>38</b>. Moreover, the first and second sealing assemblies <b>454</b>, <b>456</b> each ideally include at least one metal straight-bore type ring seal. In this manner, the single plug <b>448</b> performs the function of the two individual plugs <b>136</b>, <b>138</b> discussed above. However, only one running trip is required to install or remove the plug <b>448</b>, as opposed to separate trips to install or remove each of the plugs <b>136</b>, <b>138</b>.

The flow completion apparatus <b>410</b> preferably comprises a light-weight, non pressure-containing tree cap <b>458</b> which is installed in the tubing spool <b>12</b> above the tubing hanger <b>16</b>, and an optional debris cap <b>460</b> which is installed on the tubing spool <b>12</b> above the tree cap <b>458</b>. Referring to FIGS. <b>16</b> and <b>16</b>A-<b>16</b>C, the tree cap <b>458</b> is shown to comprise an annular body <b>462</b> which includes a number of radial sections that are secured together by a plurality of longitudinal bolts <b>464</b>. The body <b>462</b> is preferably manufactured from a non-metallic material, such as an ultra-high molecular weigh polyethylene which has a very low water adsorption rate on the order of about 0.03% in the 24 hour ASTM D570 test. This material not only makes the tree cap <b>458</b> lightweight, thereby allowing the tree cap to be installed by an ROV, but also isolates the tree cap from the cathodic protection system of the flow completion apparatus. Moreover, any longitudinal forces acting on the tree cap will be borne by the bolts <b>464</b>, thereby relieving the body <b>462</b> of this function.

Referring specifically to FIG. 16B, the tree cap <b>458</b> also comprises a collet sleeve <b>466</b> which is threaded onto the outer diameter of the body <b>462</b> near the top of the tree cap. The collet sleeve <b>466</b> includes a number of downwardly depending collet fingers <b>468</b> which are adapted to engage a corresponding groove that is formed in the upper extension <b>78</b> of the tubing hanger <b>16</b> when the tree cap <b>458</b> is landed in the tubing spool <b>12</b>. The tree cap further includes an elongated lock mandrel <b>470</b> which comprises a camming ring <b>472</b> that is attached to a lower end thereof, a plurality of locking dogs <b>474</b> which are slidably received in corresponding radial apertures that are formed in the body <b>462</b>, and a landing ring <b>476</b> which is secured by a number of lock down screws <b>478</b> in a support sleeve <b>480</b> that is threaded onto the collet sleeve <b>466</b>. Also, as shown in FIG. 16C, the tree cap <b>458</b> preferably includes a key <b>482</b> that comprises a radially extending tongue <b>484</b> which is received in a corresponding recess that is formed on the lock mandrel <b>470</b>. The key <b>482</b> serves to maintain the lock mandrel <b>470</b> in the up position until the tree cap <b>458</b> is landed in the tubing spool <b>12</b>.

When the tree cap <b>458</b> is landed in the tubing spool <b>12</b>, the landing ring <b>476</b> will land on the top of the tubing hanger locking mandrel <b>80</b> and the collet fingers <b>468</b> will enter their corresponding groove on the upper extension <b>78</b>. After the ROV turns the key <b>482</b> to release the lock mandrel <b>470</b>, the lock mandrel is pushed downward by an ROV handling tool (not shown) to force the camming ring <b>472</b> against the locking dogs <b>474</b>, which in turn will move radially outwardly against the collet fingers <b>468</b> to secure the collet fingers in their groove. The longitudinal spacing between the landing ring <b>476</b> and the collet fingers <b>468</b> can be adjusted using the lockdown screws <b>478</b>. Also, once the tree cap <b>458</b> is secured to the tubing hanger <b>16</b>, the lock down screws <b>478</b> can be tightened by an ROV torque tool <b>486</b> to firmly secure the landing ring <b>476</b> against the tubing hanger locking mandrel <b>80</b>. In this manner, the landing ring <b>476</b> will function to maintain the tubing hanger locking mandrel <b>80</b> in the locked position.

As shown in FIG. 16A, the tree cap <b>458</b> can include a conduit <b>488</b> which comprises a locking profile for an electrical connector. Thus, the tree cap can facilitate connecting an external electrical service and control line to a corresponding service and control conduit in the tubing hanger <b>16</b>. The tree cap <b>458</b> may also include an ROV hot stab <b>490</b> through which a corrosion inhibitor may be injected into the central bore <b>26</b> of the tubing spool <b>12</b> around the tree cap.

As shown in FIGS. 16B and 16C, the tree cap <b>458</b> may also comprise an annulus seal stab <b>492</b>. The seal stab <b>492</b> ideally comprises a threaded stem <b>494</b> which is received in a corresponding threaded receptacle <b>496</b> that is attached to the lower end of an actuating shaft <b>498</b>. In addition, the seal stab <b>492</b> is optimally keyed to a surrounding receptacle <b>500</b> in a manner which prevents rotation but permits longitudinal movement of the seal stab relative to the receptacle. Thus, rotation of the actuating shaft by a suitable ROV tool (not shown) will move the seal stab <b>492</b> downward into engagement with, for example, the annulus bore <b>42</b> in the tubing hanger <b>16</b>. The seal stab <b>492</b> may be a blind stab, in which event it functions to provide a backup barrier to the annulus bore <b>42</b>. Alternatively, the seal stab <b>492</b> may comprise a through bore <b>502</b> which communicates through a corresponding conduit <b>504</b> with a fluid coupling <b>506</b> that is mounted in the top of the tree cap <b>458</b>. In this manner, the pressure in the tubing annulus <b>42</b> may be monitored through the seal stab <b>492</b> and a corresponding external service and control line which is attached to the coupling <b>506</b>, or the seal stab can be used to convey gas or other fluids from an external service and control line into the tubing annulus.

Referring to FIGS. 13 and 17, the debris cap <b>460</b> is preferably a separate member which is mounted to the top of the tubing spool <b>12</b> after the tree cap <b>458</b> is installed. The debris cap <b>460</b> comprises an annular body having an outer rim <b>508</b> which is sized to fit around the outer diameter of the tubing spool <b>12</b> and an inner rim <b>510</b> which is adapted to fit closely around the upper end of the tree cap <b>458</b> and which defines an enlarged opening <b>512</b> through which the top of the tree cap <b>458</b> may be accessed. In addition, the debris cap <b>460</b> preferably includes suitable seals which are positioned between the outer rim <b>508</b> and the tubing spool <b>12</b> and between the inner rim <b>510</b> and the tree cap <b>458</b>. The seals function to keep sea water out of and corrosion inhibitor in the portion of the central bore <b>26</b> around the tree cap <b>458</b>.

The debris cap <b>460</b> is secured to the tubing spool <b>12</b> by preferably two locking pins <b>514</b>, each of which is rotatably received in a cylindrical housing <b>516</b> that is attached to the outer rim <b>508</b>. Each pin <b>514</b> includes a radially extending lug <b>518</b> which slidably engages a corresponding dogleg groove <b>520</b> that is formed in the housing <b>516</b>. In addition, the debris cap <b>460</b> includes a handle <b>522</b> which is connected to both pins <b>514</b>. When the handle <b>522</b> is in the raised position, the lugs <b>518</b> will occupy the radial outer portion of the dogleg groove <b>520</b> and the pins will be in a radially outward position. When the handle <b>522</b> is lowered, the lugs <b>518</b> will follow the dogleg groove <b>520</b> radially inwardly and thereby force the pins into engagement with an annular groove which is formed on the outer diameter of the tubing spool <b>12</b> to thereby lock the debris cap to the tubing spool.

Another embodiment of a flow completion system according to the present invention is illustrated in FIG. <b>18</b>. The flow completion assembly of this embodiment, which is indicated generally by reference number <b>610</b>, is shown to be similar in many respects to the flow completion systems <b>10</b>, <b>410</b> discussed above. However, the flow completion assembly <b>610</b> does not comprise an annulus bore <b>42</b> extending through the tubing hanger <b>16</b>. Rather, the flow completion assembly <b>610</b> includes a workover passageway <b>612</b> which extends through the tubing spool <b>12</b> from the annulus passageway <b>412</b> to above where the second annular seal <b>142</b> seals to the central bore <b>26</b>. In addition, the flow completion system <b>610</b> preferably includes a workover valve <b>614</b> for controlling flow through the workover passageway <b>612</b>. In this manner, fluids may be communicated between the tubing annulus <b>32</b> and the portion of the central bore <b>26</b> which is located above the tubing hanger <b>16</b> through the annulus passageway <b>412</b> and the workover passageway <b>612</b>.

Although the above-described embodiments of the flow completion system of the present invention preferably include two pressure-containing barriers which are associated with the tubing hanger, in circumstances where only a single pressure-containing barrier is required between the well bore and the environment, the present invention contemplates that only a single barrier may be associated with the tubing hanger. In such a flow completion system, a pressure-containing tree cap would not be required to provide a second barrier between the well bore and the environment. Rather, a simple debris cap or lightweight, ROV deployable tree cap such as disclosed herein may be employed to prevent sea water from entering the production bore above the tubing hanger and to possibly provide for fluid or electrical communication between the service and control conduits in the tubing hanger and corresponding external service and control lines. This embodiment of the flow completion system is advantageous because it does not require the use of a pressure-containing tree cap and therefore eliminates the above-mentioned problems which are associated with using such a tree cap.

It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. For example, the various elements shown in the different embodiments may be combined in a manner not illustrated above. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.